Daily driven for a year then DOA! Dreaded crank no start. Please help

[TACTICALNAYNAY]

I’ve got a 1997 XJ 4.0 with an AW4 transmission and a 231J transfer case that's giving me a ton of trouble. It started with a rough idle—kind of felt like it was only running on three cylinders. My gas mileage plummeted to about 6-8 MPG, and acceleration was super sluggish, like I was riding a turtle or something. Plus, I'd get these weird hesitations when taking off from stoplights. So, I swung by O'Reilly Auto Parts to scan for codes since I don't have a check engine light (I think it’s burnt out or just gone). The scan pointed to a bad battery temperature sensor, so I replaced that.

After disconnecting the battery to reset the PCM and clear some codes, I hooked everything back up, and at first, it started and seemed fine. But then I noticed the alternator was dead. When I crawled under the hood, it started misfiring again, and the alternator still wasn’t charging.

I spent days digging through forums, and most folks thought it could be the PCM. So, I grabbed a new one and plugged it in, after making sure to disconnect the battery and wait an hour. But then, bam! No spark, no communication from the PCM to any modules, and I couldn’t get an injector pulse on either PCM. It was like someone flipped a kill switch; it would crank, but just wouldn’t start.

I have this list of tests from a shop that checked my Jeep, but they didn’t offer anything new. They wanted about $400-$500 to rebuild my engine harness because the injector plugs had some exposed wiring. Since then, I've replaced the battery, done a bunch of OHM and voltage tests, checked the wiring, and cleaned up all the grounds and connections.

I need to get my daily driver back on the road—my shop truck just isn’t cutting it! If you need any more info, just let me know. Thanks a ton!

 

[Anak]

Post up the list of tests and the results of those tests.

 

[TACTICALNAYNAY]

Things that I have tested with incandescent test light or a voltmeter I will get the results from those tests when I get home from work;

Crankps
Cps
Coil
Spark
Voltage on injectors
Asd relay slot 87 and 30 in the PDC
all of the fuses in the PDC and Junction box

I have 5v going to all of the required sensors for start up, when I put a test light on the coil 12v supply and turn it over I loose the light, my ASD relay clicks, after that the light comes back on. When I turn the jeep over by hand I can hear all of my injectors click so I know that it is getting signal from the flywheel

 

[lawsoncl]

So it won't start at all after swapping the PCM? That could be anti-theft or the PCM isn't the right one. I'd swap the original back in and continue troubleshooting.

In your PM you said - "I have a list of things a shop did to test and diagnose the Jeep, which I can send you if needed, but unfortunately, they couldn’t provide any additional insights. They did suggest spending an extra $400-$500 to have them rebuild my engine harness because my injector plugs show some exposed wire near the pin connection."

Try wiggling and moving the harness across the fuel rail and where it crosses over to the firewall. If that makes it stumble or get better, then it's likely the wires are frayed or damaged in the wiring harness. This is usually caused by bad motor mounts, btw. You can also pull off the injector connections one-by-one while running to see if any don't affect the idle. I'd also unplug the O2 sensor and see if that helps, to rule out a bad O2 sensor.

 

[TACTICALNAYNAY]

The new pcm is compatible with my original computer and the new computer did not have the SKIM system as far as I am aware, but no it will not start with either computer I will try to wiggle the wires and see if it uncovers something more. Also I have tried to put the old pcm back in after I had the harnesses out and it was the same thing a crank no start/no spark I've been over this jeeps engine bay top to bottom left to right…. Its been down since October

 

[Anak]

In one place you say "couldn’t get an injector pulse on either PCM", in another you say "I can hear all of my injectors click so I know that it is getting signal from the flywheel"

I am confused.

Have you plugged in a noid light?

Among missing details, what is your fuel pressure? (As in a definite number, not "I pushed the Schrader valve and fuel came out".)

How has the coil been tested?

 

[TACTICALNAYNAY]

In one place you say "couldn’t get an injector pulse on either PCM", in another you say "I can hear all of my injectors click so I know that it is getting signal from the flywheel"

I am confused.

Have you plugged in a noid light?

Among missing details, what is your fuel pressure? (As in a definite number, not "I pushed the Schrader valve and fuel came out".)

How has the coil been tested?

No I don't have a noid light but I do have a test light. 49psi fuel pressure when key is on the fuel pump comes on for 2-3 seconds then shuts off. I started to hear the injectors click after I repaired the injector harness I can physically feel them click. The ignition coil was tested for ohms as well as I have used my test light backprobing while cranking, the coil tested a little out of spec the primary coil was at 1.5k Ohms and the secondarys were at 9.30k Ohms. I currently am making sure that the TPS, Crank position sensor and the map are getting 5v reference which they are.

 

[Anak]

Have you checked for a grounded out O2 sensor? Those make for an unintended kill switch.

 

[Tim_MN]

6-8 mpg's suggests O2 sensor issues.

 

[TACTICALNAYNAY]

6-8 mpg's suggests O2 sensor issues.

I'm cruising on 33's with stock gears for now, lol . The O2 sensor wires are still in the sheath, and I had the harness side of the plug totally out of the Jeep when I checked Out the complete harness

 

[TACTICALNAYNAY]

The problems only started after I put in the new computer, while I was chasing the no-charging issue. That is why I am so baffled and confused, also wiring above a stereo or some aftermarket lighting is above my pay grade so please bear with me.

 

[Tim_MN]

1991-2001 HO

A faulty Jeep 4.0 oxygen sensor can manifest as a lit check engine light, decreased fuel efficiency, rough idling, hesitation during acceleration, loss of power, black smoke from the exhaust, a sulfur-like smell from the exhaust, and potential engine misfires; essentially, the engine will not be running optimally due to an incorrect air-fuel mixture caused by the faulty sensor.

Key symptoms to watch for:
Check Engine Light: This is the most common indicator of a faulty oxygen sensor and will usually be accompanied by a diagnostic trouble code related to the sensor.
Reduced Fuel Economy: A bad oxygen sensor can lead to the engine burning more fuel than necessary, resulting in lower gas mileage.
Rough Idling: The engine may run unevenly or stumble at idle due to inconsistent fuel mixture.
Hesitation on Acceleration: You might notice a delay or hesitation when accelerating, as the engine struggles to adjust to the correct fuel mixture.
Loss of Power: In severe cases, a faulty oxygen sensor can lead to noticeable power loss.
Black Exhaust Smoke: Excessive fuel being burned can produce black smoke from the exhaust.
Sulfur Smell: A rotten egg-like smell from the exhaust could also indicate a failing oxygen sensor.

The O2 Sensor is also the main sensing element for the Catalyst and Fuel Monitors. The O2S can fail in any or all of the following manners:
slow response rate
reduced output voltage
dynamic shift
shorted or open circuits

Response rate is the time required for the sensor to switch from lean to rich once it is exposed to a richer than optimum A/F mixture or vice versa. As the sensor starts malfunctioning, it could take longer to detect the changes in the oxygen content of the exhaust gas.

The output voltage of the O2S ranges from 0 to 1 volt. A good sensor can easily generate any output voltage in this range as it is exposed to different concentrations of oxygen. To detect a shift in the A/F mixture (lean or rich), the output voltage has to change beyond a threshold value. A malfunctioning sensor could have difficulty changing beyond the threshold value.

For the HO years, 91-02, the O2 sensor has 4 wires. One wire is the 12-14 volt power that comes from the O2 sensor heater relay on the passenger side firewall (unless they moved them in later years to the relay & fuse boxes?), and that powers the internal heater in the sensor so that the sensor can work at idle, and almost immediately after start up. Loss of that power will hurt gas mileage even with a good O2 sensor.

Two black wires are grounds for the heater power and O2 signal to the ECU. A poor ground will limit current to the O2 sensors heater or cause an error in the O2 sensor output voltage read by the PCM (ECU) causing poor mileage even with a good O2 sensor. Test the 2 ground wires with power off. Test between the wire end at the sensor and the battery negative ground post. It should read less than 1 ohm.

The last wire, #4, is a signal feed wire, 0-1 volts, from the O2 sensor to the PCM (ECU) sensor. The O2 sensor is an O2 concentration sensitive variable voltage generator. At optimal O2 concentration the O2 sensor puts out 0.45 volts.

At idle that voltage should read 0.1-0.9 volts oscillating quickly back and forth roughly once every second. At 2000 rpm it should run between 0.4 and 0.5 volts max (in park). A digital meter can NOT be used for reading the O2 sensor voltage, but it can be used to test the grounds and the 12-14 volts to the heater. You must use an old style analog meter with the needle gauge on the display to see the voltage swing back and forth.

If the O2 sensor readings are not right, say they read .1 volts or .8 volts steady, you have a problem. BUT before you blame the O2 sensor make sure it has good wiring, and make sure the proper voltage is feeding it, by turning power on, engine off to read the engine off voltage feeds (12-14 on one, and ensure the ground wires (power off) reads less than 1 ohm to the battery negative post.

If any one of the grounds or the O2 sensor to ECU/PCM wires reads a high voltage chances are the wiring harness is damaged and the 12-14 volts for the O2 sensor heater is getting through a bad spot in the wires insulation and contacting a ground wire or the O2 sensor wire feed to the ECU/PCM. This is not the O2 sensors fault, but a wiring problem.


A leaky exhaust system or leaky fuel injector(s), or bad compression or leaky valves, bad plugs, wires, cap, rotor, coil, and so on, or combination of these, can also cause a lean or rich condition that gives you high or low O2 sensor readings that are not the O2 sensors fault, so try and verify those other items also before buying parts like an O2 sensor to fix your problem.

 

[TACTICALNAYNAY]

The O2 sensor and adjacent wiring checks out and looks good all the way through

 

[Da XJ nut]

my poor little 97 XJ is the same year as his doing the same thing. My PCM will not communicate with my Snap-on scanner. I have a known good OEM crank sensor, cam sensor and just did the ZJ tank upgrade with new Carquest fuel pump assembly. It has not ran since March after issues with extended crank due to low fuel psi. Mine was not allowing the voltage and fuel gauge to work before it wouldn't start and then cycle the key 10+ times the gauges would pop on and it would run. Now it's DOA with no communication and no working voltage,fuel gauges and no run with good fuel psi and haven't check spark yet. I have also tested all harness connections and wires from firewall to intake manifold. Sorry to high jack your thread. I NEED HELP TOO!!

 

[Tim_MN]

My PCM will not communicate with my Snap-on scanner.......... Now it's DOA with no communication and no working voltage, fuel gauge

These are Primary symptoms of a faulty CPS.



The most likely cause of it cranks and cranks but won't start up is the Crankshaft Pickup Sensor (CPS) located on the transmission bell housing. Often this part is also referred to as the CranKshaft Position Sensor (CKP). CPS/CKP failure is very common. The CPS/CKP can stop working with no warning or symptoms and the engine will not run or the engine may randomly stall for no apparent reason. Typical genuine Jeep CPS lifespan is about 150-200,000 miles.

Crank Position Sensors can have intermittent “thermal failure”. This means that the CPS/CKP fails when the engine gets hot, but works again when it cools back down.

Typical CPS Symptoms, (not all symptoms may be present, or occur at the same time) -
- Random stalling
- Starter cranks and cranks but engine won't start up
- Fuel gauge and voltage gauges may not work or display properly.
- You sometimes will have NoBus displayed on the odometer after 30-60 seconds.
- A failed CPS/CKP may or may not throw a CEL trouble code.
- Fuel pump should run and prime for 3-5 seconds.
- No spark at the spark plugs.

When the ignition key is first turned ON, 12 volts travels from the ignition switch to the ASD relay. The PCM provides ground to the ASD relay to energize it. The ASD relay sends the 12 volts to the primary side of the coil. If after a few seconds no signal is detected from the faulty CPS, the PCM opens the ASD relay and 12 volts power to the coil and fuel pump is removed.

If the CPS/CKP is failed sometimes the OBD-II code reader cannot make a connection to the computer or cannot read Check Engine Light/MIL codes because the CPS/CKP has failed.

Diagnostic steps to help confirm the CPS is the cause of your no-start:
-You should be able to verify a bad cps, by unplugging it, and turning the ignition key to ON. If the voltage gauge and/or the fuel gauge now displays correctly, replace the CPS.
-Unplugging and reconnecting the CPS sensor where it connect to the main harness near the back of the intake manifold usually resets the ECU and if the jeep fires right up after doing this you can bet that the CPS is faulty and needs to be replaced.
-Exchange the fuel pump relay and the ASD relay with one of the other similar ones in the PDC to eliminate these relays as the cause of the no-start. Confirm that the fuel pump runs for 3-5 seconds when you turn the ignition key to ON.
-Eliminate the NSS as a cause of no start. Wiggle the shift lever at the same time you try to start. Put the transmission in Neutral and do the same. Do the reverse lights come on when the shifter is in Reverse?
-Inspect the wires and wire connectors at the O2 sensors on the exhausts pipe. A short circuit from melted insulation or from broken O2 sensor wires can blow a fuse and the ECU/ECM will lose communication.
-Check the ground connection for the PCM. Make sure it is clean, snug, and has no corrosion.

If you buy a new CPS, get a genuine Jeep CPS or an NTK. Most auto parts stores sell cheap crappy Chinese "Lifetime Warranty" parts that are poorly manufactured from inferior materials and are often out of specification, or even failed, right out of the box. The ones that aren’t faulty often have a short service life before they fail. Always buy top quality replacement parts and genuine Jeep sensors. Numerous threads detail long and frustrating searches for a "problem" that ended up being cured simply with genuine Jeep repair parts. Buy good quality repair parts and genuine Jeep sensors for best results.

You must also perform basic trouble shooting of the start and charge systems. Remove, clean, and firmly reconnect all the wires and cables to the battery, starter, and alternator. Look for corroded or damaged cables and replace as needed. Do the same for the grounding wires from the battery and engine to the Cherokee's frame/body. Jeeps do not tolerate low voltage or poor grounds and the ECM/ECU will behave oddly until you remedy this.
Crankshaft Position Sensor Connector (CPS/CKP)



CPS Testing
TESTING PROCEDURE 1991 –2001 4.0L H.O. engines
1. Near the rear of fuel rail, disconnect sensor pigtail harness connector from main wiring harness.
2. Place an ohmmeter across terminals B and C or 2 and 3 (See Image). Ohmmeter should be set to the 10K-or-2OK scale for this test.
3. The meter reading should be open (infinite resistance). Replace sensor if a low resistance is indicated.